As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for secondary batteries as an energy source for the mobile devices has also sharply increased. Accordingly, much research on secondary batteries satisfying various needs has been carried out.
In terms of the material for batteries, the demand for lithium secondary batteries, such as lithium ion polymer batteries, having high energy density, discharge voltage, and output stability is very high. In terms of the shape of batteries, the demand for prismatic batteries or pouch-shaped batteries, which are thin enough to be applied to products, such as mobile phones, and may be used as batteries for a battery module manufactured by stacking the batteries with high integration is very high.
An electrode assembly for secondary batteries, in which battery reaction is generated, is generally configured to have a structure in which a cathode sheet having a cathode active material applied thereto, an anode sheet having an anode active material applied thereto, and a separator are impregnated with an electrolyte. Based on the structure thereof, an electrode assembly for secondary batteries is classified as a jelly-roll type (wound type) electrode assembly and a stacked type electrode assembly. The jelly-roll type electrode assembly or the stacked type electrode assembly is mounted in a prismatic metal case to manufacture a prismatic battery.
Generally, a prismatic battery is manufactured by mounting an electrode assembly in a prismatic metal case, loading an upper insulator on the open upper end of the prismatic metal case, connecting a cap plate to the upper insulator by welding, injecting an electrolyte through an electrolyte injection port, and sealing the electrolyte injection port.
At this time, a sealed portion of the battery case, in which the battery cell is mounted, is partially bent, and an insulative tape is attached to it or a barcode is printed on it. However, this process is very complicated.
Also, when external impact is applied to a battery pack, a protection circuit module (PCM) may be damaged or dimensional stability of the battery pack may be greatly lowered due to the use of the insulative tapes, which exhibit low mechanical strength.
In order to solve the above problems, therefore, surface treatment may be carried out with respect to the battery case, or a cover having predetermined strength may be provided at the battery case. In this case, however, welding may not be satisfactorily carried out due to the surface treatment carried out with respect to the battery case when the cap plate is connected to the open upper end of the battery case by laser welding.
Meanwhile, various kinds of combustible materials are contained in a lithium secondary battery. As a result, the lithium secondary battery may be heated or explode due to overcharge of the lithium secondary battery, overcurrent in the lithium secondary battery, or other external physical impact applied to the lithium secondary battery. That is, the safety of the lithium secondary battery is very low. Consequently, safety elements, such as a positive temperature coefficient (PTC) element and a protection circuit module (PCM), to effectively control an abnormal state of the lithium secondary battery, such as overcharge of the lithium secondary battery or overcurrent in the lithium secondary battery, are loaded on a battery cell in a state in which the safety elements are connected to the battery cell.
Generally, the PCM is electrically connected to the battery cell via conductive nickel plates by welding or soldering. That is, nickel plates are connected to electrode tabs of the PCB by welding or soldering, and the nickel plates are connected to electrode terminals of the battery cell by welding or soldering. In this way, the PCM is connected to the battery cell to manufacture a battery pack.
It is required for the safety elements, including the PCM, to be maintained in electrical connection with the electrode terminals of the battery cell and, at the same time, to be electrically isolated from other parts of the battery cell. To this end, a plurality of insulative mounting members or other parts is necessary, which complicates an assembly process of the battery pack. In particular, the sum of the height of the PCM and the height of the insulative cap generally reaches 3 mm with the result that a space necessary to receive the battery cell is reduced.
Consequently, there is a high necessity for a bus bar assembly having a novel structure that is capable of improving durability of a battery case and easily performing laser welding while solving the above-mentioned conventional problems.